Unsaturated polyester resins are used to make composite (i.e., fiber reinforced plastic or FRP) structures. A component of some composite structures known as gel coat is commonly used to provide protection and beautification of the overall composite structure. The majority of gel coats are based on linear, unsaturated polyesters derived from maleic anhydride, diacids such as adipic acid, isophthalic acid and phthalic anhydride, and diols such as 1,2-propanediol and 2,2-dimethyl-1,3-propanediol. The unsaturated polyester resin is typically dissolved in and copolymerized with a ethylenically unsaturated monomer, i.e., styrene, to form the final cured resin. Varying degrees of UV weathering and water resistance (i.e., osmotic blister resistance) may be obtained by carefully selecting the types and ratios of monomers and processing them to a particular molecular weight or other parameter using techniques that are well known to polymer chemists. Alternatively, additives may be included in the resin to improve its UV weathering or water resistance.
The preparation of gel coats with good osmotic blister resistance in combination with other desirable properties has been difficult. For example, good osmotic blister resistance can be obtained by incorporating aromatic diacids such as, for example, isophthalic acid (abbreviated herein as “IPA”) into the unsaturated polyester resin gel coat. Aromatic molecules adsorb UV energy, however, and gel coats with high aromatic content can experience accelerated loss of specular gloss and increase yellowing upon UV exposure. Substituting various aliphatic dicarboxylic acids for IPA can improve UV weathering, but generally reduces osmotic blister resistance and often may cause poor polymer solubility in styrene. Solubility in styrene is needed for proper application and cure of the final composite part. New, unsaturated polyester resins are needed, therefore, that provide good UV weathering properties in conjunction with osmotic blister resistance.